Endocytosis is a general term defining processes, by which a cell can import and/or export selected extracellular species, such as molecules, viruses, particles and microorganisms and target them to specific organelles within a cytoplasm. Endocytosis can occur through a variety of pathways including clathrin-mediated and caveole-mediated endocytosis; phagocytosis; clathrin- and caveole-independent endocytosis. A particular endocytosis pathway may depend on the size and nature of the extracellular cargo, see e.g. Conner S. D. and S. L. Schmid. 2003. Nature 422: 37-44. For instance, caveole-mediated endocytosis is assisted by the activation of caveole, plasma membrane invaginations, with a characteristic size of 50-60 nm; clathrin-mediated endocytosis requires the concentration of transmembrane receptors and their bound ligands on the plasma membrane leading to the formation of vesicular cages with a characteristic size up to few hundreds microns (100-500 nm); phagocytosis involves specific cell-surface receptors and signaling cascades with the formation of cell membrane protrusions that eventually envelope the external micrometer cargo (>1 μm). Clathrin- and caveole-independent endocytosis can be associated with the formation of invaginating vesicles smaller than 100 nm.
Particle endocytosis can be of fundamental importance in several fields, such as virology, drug and gene delivery and in nanotoxicology, see e.g. Marsh M. and A. Helenius. 2006. Cell 124:729-40; Vasir J. K., and V. Labhasetwar. 2006. Expert Opin. Drug Deliv. 3:325-344; Oberdorster G., E. Oberdorster, J. Oberdorster. 2005. 113:823-39.
For nanosized particles, both natural, such as enveloped viruses, or artificial, such as biomimetic particulates, the most effective internalization mechanism can be a receptor-mediated endocytosis, in which molecules (ligands) distributed over the particle surface bind to countermolecules (receptors) expressed over the cell membrane, which can eventually bend to invaginate the foreign cargo, see e.g. Marsh M. and A. Helenius. 2006. Cell 124:729-40; Smith A. E., A. Helenius. 2004. Science 304:237-42. These receptors can be collected at the site of invagination by surface diffusion, without which endocytosis would not occur or would occur over a much longer time scale.